1. Field of the Invention
The present invention relates to a method for nondestructively measuring and quantitatively determining deterioration in materials accompanying neutron irradiation, and the like, in a ferromagnetic construction material, or in a structure comprised of such materials.
2. Description of the Related Art
Conventional nondestructive inspection methods for aged material deterioration had generally aimed at investigating of initiation and growth of cracks in the material in almost every case. And thus, the direction of development in present nondestructive inspection methods lies in finding out produced cracks as minute as possible. Accordingly, with such a conventional nondestructive inspection method, it is practically impossible to inspect nondestructively aged deterioration of materials before the initiation of cracks.
By the way, it is generally considered that aged deterioration in a nuclear reactor pressure vessel goes on by combining precipitation of copper atoms, a dislocation loop etc, due to metal fatigue and neutron irradiation.
Another type of previous method is known for nondestructively determining deterioration of material strength due to aging of ferromagnetic construction materials or structures comprised of such construction materials. In this determining method, the coercive force and magnetic susceptibility in the range approaching to saturation of a determining object are measured.
Moreover, Japanese Patent Laid-open No. 2001-021538 discloses about aged deterioration due to metal fatigue of materials before the initiation of cracks. As described in this document, conventionally, the following nondestructive inspection method is known. That is, in the inspection method, the coercive force Hc and susceptibility coefficient c (Hereinafter referred to as a strength parameter c.) are measured. Then, from the strength parameter c, aged deterioration of strength in ferromagnetic construction materials or structures comprised of such the construction materials is determined.
Then, the inventor thought that if a nondestructive inspection for aged deterioration accompanying the change in brittleness of materials could be carried out by combining with the nondestructive inspection for aged deterioration accompanying the change in strength of materials, it would lead to much more improvement in the safety of a nuclear reactor pressure vessel. So the inventor focused attention on nondestructive inspection for determining aged deterioration accompanying the change in brittleness of materials.
However, it was impossible to apply the above-mentioned conventional nondestructive inspection method for aged deterioration of material strength to the nondestructive inspection method for aged deterioration accompanying the change in brittleness of materials due to precipitation of copper atoms and so on.
That is, conventional measuring objects are dislocations produced by metal fatigue. In such dislocations, anisotropic strain fields exist in the interior of materials. Therefore, it was possible to inspect nondestructive aged deterioration of materials by the conventional method measuring the coercive force Hc, because aged deterioration of material strength has much effect on the coercive force Hc.
On the other hand, in aged deterioration accompanying the change in brittleness of materials due to precipitation of copper atoms and so on, measuring objects are defects. The defects are atomic vacancies or interstitial atoms produced by irradiating neutron etc, or precipitation by heat treatment etc., and so on. In such defects, strain fields do not always exist in the interior of materials.
Therefore, aged deterioration accompanying the change in brittleness of materials hardly have much effect on the coercive force Hc. Accordingly, it was impossible to apply the method determining the coercive force Hc among above-mentioned nondestructive inspection methods for aged deterioration of material strength to determining of aged deterioration accompanying the change in brittleness of materials. Also, it was impossible to determine aged deterioration accompanying the change in brittleness of materials even by means of the conventional method obtaining the strength parameter c of the determining objects.
Accordingly, it is difficult to determine quantitatively embrittlement results from increase of precipitation of copper atoms and atomic vacancies.
Therefore, a new determining factor for examining quantitatively such embrittlement was necessary.
It is therefore a primary object of the present invention to provide an improved measurement method for nondestructively determining aged deterioration of ferromagnetic construction materials, which advantageously eliminates the above-mentioned problems of the prior art.
One aspect of the present invention resides in the method for nondestructively determining aged deterioration of ferromagnetic construction materials by quantifying the change in brittleness due to aging of the materials. The determining method according to the present invention includes the following steps.
One of the steps is to suppose to acquire an embrittlement coefficient b by measuring a magnetic susceptibility "khgr"b of the determining ferromagnetic construction material under a magnetic field having a predetermined magnetic field intensity H over a magnetic coercive force Hc of the determining material, and calculating an embrittlement coefficient b of the determining material by putting the magnetic field intensity H and the measured magnetic susceptibility "khgr"b of the determining material into an equation,
b="khgr"bH2xe2x80x83xe2x80x83(1).
Another one of the steps is to obtain a correlation between an embrittlement coefficient b and a referenced embrittlement factor of the same kind of ferromagnetic construction materials as the determining material previously, the value of the referenced embrittlement factor changes corresponding to the change in brittleness of the same kind of ferromagnetic construction materials.
Further one of the steps is to obtain the values of the embrittlement coefficient b of the determining ferromagnetic construction material in the initial state and the deteriorated state by aging.
Further one of the steps is to determine the values of the referenced embrittlement factor corresponding to the values of the embrittlement coefficient b respectively, based on the correlation.
Further one of the steps is to quantify the change in brittleness due to aging of the determining ferromagnetic construction material by comparing the values of the referenced embrittlement factor.
The principle of the present invention will be described below with reference to experimental test data. To clarify the correlation between the mechanical property and the magnetic property of steel materials, test pieces consisting of polycrystalline pure iron (99.992% purity) involving copper atoms (1.5 wt/%) were used. By heat treatment of test pieces in various temperatures, the copper atoms are precipitated in the test pieces.
By above-mentioned treatment of the test pieces, a precipitating quantity of the copper atoms and the size of precipitates can be changed corresponding to the change of temperatures and time in heat treatment.
By the way, it is known that copper atoms are precipitated at the heat treatment temperature, that is, the aging temperature, from 445xc2x0 C. to 650xc2x0 C., and such precipitation of cupper atoms in materials is related to hardness of materials.
In this way, precipitation of copper atoms is related to hardness in materials because the hardness of steel materials increases by the precipitation of copper atoms preventing the movement of dislocations.
Then, in this experiment, as a result of test pieces heat-treated at each temperature (aging temperatures: 455xc2x0 C., 550xc2x0 C., 650xc2x0 C.), as shown in FIG. 4, the correlation of heat treatment time (minute) and hardness (Vickers hardness Hv) was obtained in each temperature. Here, in FIG. 4, it is plotted in solid triangles (▴) about the test piece heat-treated at 455xc2x0 C., it is plotted in solid circles (xe2x97xaf) about the test piece heat-treated at 550xc2x0 C., and it is plotted in solid diamonds (♦) about the test piece heat-treated at 650xc2x0 C., respectively.
According to this result, for example, in the test piece heat-treated at 455xc2x0 C., it turns out that precipitation of copper atoms progresses most in aging time for 2.0xc3x97103 to 7.0xc3x97103 minutes.
In addition, in the above-mentioned experiment, the copper atoms are precipitated by heat treatment in the test pieces. This is because it is generally thought that, when a neutron is irradiated in a pressure vessel, copper atoms, which migrate in the inner material composing of the pressure vessel, are precipitated and the precipitates embattle the pressure vessel.
In addition to this reason, there is also an idea that dislocation loops in the material are induced by neutron irradiation in a pressure vessel and these dislocation loops contribute to the embrittlement of the pressure vessel. Therefore, in the above-mentioned experiment, test pieces are heat-treated as what is replaced with the neutron irradiation to the pressure vessel leading to precipitation of copper atoms, and, thereby, the copper atoms in the material are precipitated.
FIGS. 5 and 6 are explanation diagrams showing the hysteresis loop obtained from the loop test of the test pieces, in which the copper atoms are precipitated by heat treatment. Here, FIG. 5 shows the change of the hysteresis loop characteristics accompanying the copper precipitation by heat treatment in aging time (0 min., 30 min., 3.0xc3x97102 min., 2.0xc3x97103 min., 7.0xc3x97103 min.) under aging temperature of 455xc2x0 C. Also, FIG. 6 shows the change of the hysteresis loop characteristics accompanying the copper precipitation by heat treatment in aging time (0 min., 30 min., 1.0xc3x97102 min., 2.0xc3x97102 min., 1.0xc3x97103 min.) under aging temperature of 550xc2x0 C. In the hysteresis loops obtained by hysteresis loop characteristic test of both of FIGS. 5 and 6, significant change is not seen depending on the change of aging time.
However, on study of the inventor, when analysis is performed as below, it was proved that the change (change of brittleness) in the precipitation state of copper atoms in the material accompanying the change of aging conditions (aging time, aging temperature) based on the hysteresis loop could be expressed quantitatively.
From the hysteresis loop as shown in FIG. 5, the relation of the logarithm of magnetic susceptibility "khgr"b (=flux density B (Gauss)/magnetic field intensity H (Oe)) to the logarithm of the magnetic field intensity H is plotted. Thereby, the relationships as shown in FIGS. 7 and 8 are obtained.
Here, FIG. 7 illustrates the relationship in a test piece aged for aging time 3.0xc3x97102 minutes under aging temperature of 455xc2x0 C., and FIG. 8 illustrates the relationship about a test piece aged for aging time 2.0xc3x97103 minutes under aging temperature of 455xc2x0 C.
In addition, in each of FIGS. 7 and 8, the logarithm is illustrated using common logarithm (log base 10), and the straight line of gradient of xe2x88x922 being best match with related diagram is illustrated. And from the relationships (the straight lines of gradient of xe2x88x922) shown by FIGS. 7 and 8, the relationship as shown with the next equation is obtained.
log ("khgr"b)=log (b)xe2x88x922 log (H)xe2x80x83xe2x80x83(2).
And from this equation (2), the next equation is obtained.
xe2x80x83"khgr"b=b/H2xe2x80x83xe2x80x83(3).
This equation can change into the equation (1) described previously. Therefore, log (b) is determined from the gradient of xe2x88x922 (a straight line of the equation (2)) shown in FIGS. 7 and 8. Therefore, the embrittlement coefficient b can be obtained from the values of the log (b).
FIGS. 9, 10 and 11 are explanatory diagrams illustrating, by comparing with Vickers hardness Hv, the relation between the embrittlement coefficient b and aging time obtained from above-mentioned means. Here, in FIG. 9 is shown the relationship under aging temperature of 455xc2x0 C., in FIG. 10 is shown the relationship under aging temperature of 550xc2x0 C., and in FIG. 11 is shown the relationship under aging temperature of 650xc2x0 C.
According to these relationships, it became clear by experiment of the inventor that the phenomenon that the value of the embrittlement coefficient b decreases and takes the local minimum and the phenomenon that Vickers hardness Hv increases and takes the local maximum correspond well according to aging time about all of FIGS. 9 to 11. Then, if the relation between the embrittlement coefficient b and Vickers hardness Hv is illustrated from the related diagram expressed with FIGS. 9 to 11, the distribution of the values shown in FIG. 12 will be obtained, and the correlation as shown by the curve in FIG. 12 will be obtained.
In addition, in FIG. 12, it is plotted in solid triangles (▴) about the test piece heat-treated at 455xc2x0 C., it is plotted in solid circles (xe2x97xaf) about the test piece heat-treated at 550xc2x0 C., and it is plotted in solid diamonds (♦) about the test piece heat-treated at 650xc2x0 C., respectively.
By the way, according to the above-mentioned FIG. 4, it turns out that Vickers hardness Hv shows it gets to the local maximum (maximum) near for 5.0xc3x97103 minutes at the aging temperature of 455xc2x0 C., the local maximum (maximum) near for 2.0xc3x97102 minutes at the aging temperature of 550xc2x0 C., the local maximum (maximum) near for 10 minutes at the aging temperature of 650xc2x0 C., respectively. This is based on the following reason. That is, copper atoms are precipitated with the help of atomic vacancies and the precipitate density gets to a peak in the initial stage of the precipitation. And when the test piece is aged further more, the copper precipitate grows up gradually and the average distance of precipitates becomes large. And Vickers hardness Hv recovers. Accordingly, Vickers hardness Hv degrades. This reason indicates that when Vickers hardness Hv gets to a peak, brittleness will be highest (embrittlement will progress most).
From above-mentioned reason, it can turn out that Vickers hardness Hv changes corresponding to the change in brittleness of a ferromagnetic structure material, and such Vickers hardness Hv can be used as a referenced embrittlement factor. Therefore, concerning the ferromagnetic construction materials as the object for determining aged deterioration, the values of the embrittlement coefficient b of the material in the initial state and the aged state are obtained from the relationship as shown in FIG. 12. Then, values of the referenced embrittlement factor in the initial state and the aged state corresponding to the values of the embrittlement coefficient b are obtained from the correlation. Finally, the values of the referenced embrittlement factor are compared with each other. Thus, it is possible to determine quantitatively the change (extent of embrittlement) in brittleness due to aging of the ferromagnetic construction materials with the object for determining aged deterioration.
That is, according to the method of the present invention, the method is performed by using test pieces of ferromagnetic materials of the same kind as the ferromagnetic structure material by which the non-destructive test of aged deterioration is carried out previously.
Here, the correlation between the embrittlement coefficient b and a referenced embrittlement factor (Vickers hardness Hv in FIG. 12) for example as shown by the curve in FIG. 12 is determined, and the correlation is defined as a referenced correlation of the structure material.
By the same as the above-mentioned method, it is possible to obtain the embrittlement coefficients b in the initial state and the aged state of a ferromagnetic construction material, or of a structure (for example, a reactor pressure vessel etc.) comprised of such a material from the hysteresis loop obtained by performing hysteresis loop test. Then, the values of a referenced embrittlement factor corresponding to the embrittlement coefficients b can be obtained from the aforementioned referenced correlation. By comparing the values of the referenced embrittlement factor, the extent of brittleness in the measured structure can be determined quantitatively.
Moreover, according to the conventional method measuring Vickers hardness, even if it can acquire the information about the surface of materials, the information about brittleness in the interior of determining ferromagnetic structure materials cannot be acquired. On the contrary, according to the method of the present invention, the embrittlement coefficient b is obtained based on the hysteresis loop including the information about the interior of materials. Hereby, the present invention has an advantage that the information about the whole material also including the interior of a measured ferromagnetic structure material can be acquired against the conventional method.
In addition, in case of setting instruments (exiting coil, detecting coil) to a reactor pressure vessel etc. as the determining object, a hysteresis loop characteristic test can be performed, depending on the form of a measuring object, by winding directly coil round a determining object or by applying the magnetic yoke with winding coil to the measuring object. And from the obtained hysteresis loop, it is possible to obtain the embrittlement coefficient b.
That is, if a structure material is radiated by neutrons over the long term, vacancies in the interior of materials will increase, consequently, copper atoms will be precipitated and embrittlement in the structure material will advance. According to the method of this invention, it is possible to determine correctly the extent of substantial embrittlement due to such precipitation of copper atoms etc., and it is possible to determine nondestiuctively aged deterioration of materials.
Moreover, according to the method of this invention, it is possible to combine the above-mentioned former method by which aged deterioration of the material strength can be measured nondestructively and quantitatively by obtaining strength parameter c. If it does in this way, it can ask not only for the material strength but also for the change in brittleness quantitatively. Accordingly, it is possible to check simultaneously the change of the dislocation density accompanying metal fatigue, neutron irradiation, etc., and aged deterioration accompanying the material strength and the change in brittleness can be evaluated. In addition, the above-mentioned strength parameter c is defined by next equation:
c="khgr"cH3xe2x80x83xe2x80x83(4).
Especially, it is supposed that aged deterioration of materials due to neutron irradiation in a reactor pressure vessel will advance by correlating atomic vacancies, interstitial atoms and increase of dislocation loops, etc. mutually.
Therefore, it is possible to obtain the embrittlement coefficient b in the measuring method for aged deterioration of the ferromagnetic structure material by the present invention, and values of strength parameter c in the conventional measuring method for aged deterioration of strength of the ferromagnetic structure material by above-mentioned equation (1) or equation (4) from the hysteresis loop, respectably.
So, it is possible to obtain the deteriorated information about strength and brittleness of materials respectably, and it is possible to separate increase of atomic vacancies, interstitial atoms and copper precipitation, etc. from deterioration due to dislocation loops in material strength.
Also, as for the nondestructive determining method for aged deterioration accompanying the change in brittleness of ferromagnetic construction materials in the present invention, the referenced embrittlement factor may be hardness. According to the present invention, a hardness corresponding to the value of the embrittlement coefficient b can be obtained from the next equation showing the correlation between embrittlement coefficient b and hardness:
Hv=f(b)xe2x80x83xe2x80x83(5).
And as mentioned above, it is known that hardness corresponds to the change in brittleness of materials. Accordingly, the quantitative value of the change in brittleness of materials can be calculated certainly from Vickers hardness Hv corresponding to the embrittlement coefficient b.